1. Field of the Invention
This invention relates to semiconductor devices, and more particularly to MOS type semiconductor devices such as MOS FETs or MOS logic devices.
2. Description of the Prior Art
The MOS type transistor heretofore has been said to have shortcomings of slow operating speed and small output current handling capabilities. Hence, the MOS transistors is disfavored as compared to the bipolar transistor in the fields where the high speed or the large current handling capability is required. For improving these characteristics in the MOS transistor, making the channel length between source and drain short is the most effective means, and various attempts for shortening the channel length have been tried. Where a fabrication method utilizing photolithography is used, for attempting to shorten the gate length, about 1 to 2 .mu.m is a shortest limitation practically attainable due to inherent nature of light diffractions. Also, since the spreading in resulted gate length directly causes the spreading in the mutual conductance gm, a very high precision is required in the photolithography. Although even those patterns with sizes under 1 .mu.m can be processed with the electron-beam lithography or the X-ray lithography, it will take still a long time that those processes are put to practical uses like as the conventional photolithographic process is now widely used in practice.
A method not depending on photolithography, has been developed in which two different kinds of impurities of mutually opposite conductivity types are diffused through a single diffusion mask and the channel length is defined with the difference in their diffusion depths. However, since the threshold value of the MOS type transistors varies with the impurity concentration, a precise control of the impurity distribution was required. Particularly, if a shallow diffusion is performed for obtaining the short channel length, the impurity distribution becomes very steep and hence the threshold value changes greatly with a slight variation in the distrubution of two different kinds of impurities. Thus, there exists a difficulty in controlling the threshold value. This is due to its defining procedure of the gate length wherein two different kinds of impurities must be controlled at the same time.